8:25 AM 2/27/2019 — In the event recorded by Saint Mark in today's Gospel lesson, the Blessed Apostle John has seen some guy casting out demons in our Lord's name; and, because this man isn't a member of the apostolic “club,” he wants our Lord to stop him. Our Lord's correction of John does not surprise us, and His rebuke to the Apostle is what we've come to expect: “…Whoever is not against us is for us (9: 40 NABRE]).
Some might be tempted to view this Gospel lesson as nothing more than an appeal to diversity—which is, of course, a hot topic these days—with the takeaway being how we must except everyone's unique gifts, and not exclude anyone because they're not like us, or speak our language, or see things our way. And all of that's very true. Fill in the blanks, blah, blah, blah.
But I think there’s a much more important lesson buried in this brief passage, which is hidden in the very first line of today's lesson, if you know enough to read it between the lines: “John said to him, 'Teacher, we saw someone driving out demons in your name, and we tried to prevent him because he does not follow us'” (v. 38 NABRE). Notice how he words it: not “he does not follow You,” but “he does not follow us.” The Apostles have allowed themselves to get so caught up in the excitement of the ministry that they're beginning to mistake their own will for the will of God; so, this is a problem our Lord has to nip in the bud, and He does.
Saint John Paul II had a general rule he used for judging whether someone's apostolic activity was helpful or harmful to the Church. Recognizing “the primacy given to the call of every Christian to holiness,” he said that the best indicator of the authentic nature of someone's activity for Christ was that it proclaimed and defended…

…the Catholic faith, embracing and proclaiming the truth about Christ, the Church and humanity, in obedience to the Church's Magisterium as the Church interprets it. For this reason every association of lay faithful must be a forum where the faith is proclaimed as well as taught in its total content.**

The two important words in that quote are “obedience” and “total”: obedience to the Church which alone has the authority, given to her by Christ Himself, to teach in His Name; and, total in proclaiming the whole of the truth taught by that Church, and not just the parts of the Catechism with which one personally agrees.
When the non-Apostolic exorcist was casting out demons and the Apostles wanted him stopped, what was it that indicated to our Lord that he was OK, other than the fact, of course, that our Lord is God and knows everything? Listen to the first verse again: “John said to him, 'Teacher, we saw someone driving out demons in your name…'” (Mark 9: 38 NABRE). There's the indicator right there! The man was doing this in the Name of our Lord, not in his own name for his own benefit. Whether the Apostles were tinged with jealously, or genuinely concerned for the integrity of the true faith we'll never know, and it's probably unfair of us to ascribe a motive to their attitude, as it could very well be the latter; but, the motive of the non-Apostolic exorcist is clear: everything he's doing is in the Name of the Lord. And even if, by circumstance, it had turned out that he was making some mistakes or saying a few things here or there that needed correction, that wouldn't have invalidated his entire ministry, because another indicator that his ministry is valid would then have been his willingness to accept correction from the Church. Nobody who works for or in the Church, priests included, is perfect; but, as our Lord Himself says, “There is no one who performs a mighty deed in my name who can at the same time speak ill of me” (v. 39). And, if you look at the Greek text of Mark, it's possible to translate that verse in the form of a question: “Is it likely that someone who does a deed in my name will speak ill of me?”*** Not very. And what's so scary about more people doing things in the Holy Name of Jesus?
When Joshua complained to Moses in the Book of Numbers about two elders who were prophesying on their own, Moses threw up his hands and shouted, “Would that all the people of the Lord were prophets! Would that the Lord might bestow his spirit on them all!” (Number 11: 29 NABRE). And wouldn't that be something: if every baptized Catholic became an apostle for the Church and the faith? But isn't that exactly what we are all called to be?

* Born in Assisi in 1838, Gabriel joined the Passionist Institute and became a veritable Apostle of Our Lady's Sorrows. Known for his irreproachable life, he died at the age of twenty-four, and many miralces were attributed to his intercession. He was canonized by Pope Benedict XV, who gave him as a patron to young people.